Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices for lower power and, in the case of Si, lower frequency applications. However, these more familiar semiconductor materials may not be well suited for higher power and/or high frequency applications, for example, due to their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 for GaAs at room temperature) and/or relatively small breakdown voltages.
In light of various difficulties in the fabrication/operation of Si and GaAs devices for high power, high temperature and/or high frequency applications, interest has turned to wide bandgap semiconductor materials such as silicon carbide (2.996 eV for alpha SiC at room temperature) and the Group III nitrides (e.g., 3.36 eV for GaN at room temperature). These materials, typically, have higher electric field breakdown strengths and higher electron saturation velocities as compared to gallium arsenide and/or silicon.
A device of particular interest for high power and/or high frequency applications is the High Electron Mobility Transistor (HEMT), which is also known as a modulation doped field effect transistor (MODFET). In a HEMT device, a two-dimensional electron gas (2DEG) may be formed at the heterojunction of two semiconductor materials with different bandgap energies. The smaller bandgap material may have a higher electron affinity than the wider bandgap material. The 2DEG is an accumulation layer in the undoped (“unintentionally doped”) smaller bandgap material, and can contain a relatively high sheet electron concentration, for example, in excess of 1013 carriers/cm2. Additionally, electrons that originate in the wider bandgap semiconductor may transfer to the 2DEG, allowing a relatively high electron mobility due to reduced ionized impurity scattering. This combination of relatively high carrier concentration and relatively high carrier mobility can give the HEMT a relatively large transconductance, and may provide a performance advantage over metal-semiconductor field effect transistors (MESFETs) for high-frequency applications.
High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN) material system can generate large amounts of RF power due to a combination of material characteristics, such as relatively high breakdown fields, relatively wide bandgaps, relatively large conduction band offset, and/or relatively high saturated electron drift velocity. A major portion of the electrons in the 2DEG may be attributed to polarization in the AlGaN.
One of the continuing challenges to fabricating HEMT devices having ideal operational characteristics is maintaining high conductivity channel regions during the high temperature processes typically used during their fabrication processes.